Common problems with bicycle wheels

Common problems with bicycle wheels
Richard’s Blog

 

Common problems with bicycle wheels

 

Cracks appear, spokes need attention

 

Posted 9 August 2012

 

Simon has been having wheel trouble, manifesting as brake block rub on his Trigon carbon-fibre rim wheels and a woozy, soft, flexible sensation from the rear when riding his Mavic Ksyrium SLs – so I took a look.
 
The brake rub ‘issue’ was easily resolved. A sprinter by temperament and physical gifts, he was unhappy with the stiffness under hard acceleration of his Trigons and, in an effort to improve matters, tightened the spokes. Equally, or so he says. While it cured, or at least alleviated, the apparent lack of stiffness, this intervention also had the effect of pulling the back wheel’s rim over.

 

A fine example of a fatigue crack

 

A quick check with the dishing tool showed it to be offset a good 2mm to the non-drive side, which is what you’d expect in the circumstances. In a ‘dished’ wheel, where the rim does not lie exactly centred between the two hub flanges, the spokes on the non-dished side – non-drive in the case of a rear wheel for derailleur gears – have a more effective axial, or sideways, pull on the rim thanks to the angle at which they act on it. This means, if there is the same number on each side, that they need less tension than those on the drive side to create the same axial pull. Otherwise, the rim would be pulled across to sit equally between the flanges…
 

Another fine crack

 

By applying the same additional amount of turn to all the rear wheel’s spoke nipples, Simon had effectively increased the relative pull of the non-drive side spokes, pulling the rim over so that it was closer to that side’s brake block. Of course, had he centred the blocks, he’d have avoided the rub but the tyres would not have been in line. Unless both front and rear rims happened to be offset to the same side by the same amount.
 
A few minutes spent adding tension to the drive side spokes sorted the dishing issue, while a few more corrected the radial ‘hop’ that had somehow found its way into the rim. Say no more, eh?
 
On inspection, the rear Ksyrium wheel proved to be less easily sorted. Introduced in 1999 and regularly upgraded by Mavic over the years since, these hoops are legendary for their durability in normal use. The aluminium spokes – an industry first – sit in special nipples that thread directly into the rim, thereby avoiding the need to create a thread on the spoke itself.
 

Otherwise, the wheels work in the conventional way, with the spokes under tension keeping the rim where it is wanted. Spoke tension is high, especially on the rear wheel’s non-drive side, despite Mavic’s use here of ‘Isopulse’ radial lacing in order to even-up tension with the non-drive spokes.
 

Si’s wheel dates from 2007 and has seen plenty of race and training camp action. It has had a hard life. And the rim has reached the end of its days. Extending either side of all but one of the drive-side rim drillings was a crack of greater or lesser extent.
 

Eyelet pulled through rim

 

These cracks, of course, were the cause of the sensation of imprecision from the rear wheel. The angled faces of the thread – 60deg in a metric thread – create a ‘wedging’ effect under spoke tension that pushes apart the sides of the female thread. Any additional tension experienced by the spokes – as, for example, when accelerating out of the saddle and generating lateral force on the rim – tries to pull the nipple further through the thread. Now in two halves and weakened by the crack, the female thread gives way a little and the nipple pulls through a little further, reducing spoke tension and the control exerted by the spoke in the rim.
 
Of course, spoke tension is what created the stress in the rim that started the cracks in the first place and, like all such fatigue cracks, repeated cyclic variation in the stress is what made them grow. Spoke pull tries to ‘bow’ inwards the hub-facing wall of the rim, putting the hub-facing exterior surface in tension.
 
With the Ksyrium wheel in question, it is probable that the sharp edges of the female thread – Mavic calls the technology ‘Fore’ – offered an initiator for the cracks, but all rims in wheels with tensioned spokes are susceptible. By coincidence, I happen to be in the process of replacing a conventional rim damaged after hitting a pothole; the force of the impact was sufficient not only to depress the rim where it hit the hole but to push it outwards at a point either side – this process necessary to accommodate the displaced rim material. Here, in both cases, the eyelet had been pulled through the rim by the nipple, leaving a short crack. It may have happened in an instant rather than over a period of several years, but the same directional loading on the rim had the same result.
 
Should the Ksyrium wheel be ridden? It could be ridden, of course, and had been in use right up to the day it was checked and found unsafe. ‘Unsafe’ is the word; while unlikely to collapse in regular riding, the weakened rim might do so if subjected to an extraordinary load such as one of Si’s famously explosive sprints. So, time to fit a new one. Watch this space.
 





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